A web framework for building web platforms instead of websites/apps, where a web platform provides a plugin architecture for integrating untrusted and mutually untrusting 3rd party apps/plugins/etc (like Facebook & Twitter plugins, etc.)
See GitStar [1] for a simple implementation - a code repository like GitHub except where all functionality is implemented by untrusted 3rd party plugins.
Code at github [2] and hackage [3].
[1]:http://gitstar.com/
[2]:https://github.com/scslab/hails
[3]:http://hackage.haskell.org/package/hails